1. Field of the Invention
The present invention relates generally to a multi-hop relay cellular system, and more particularly, to an apparatus and method for managing connection identifiers (CIDs) in a multi-hop relay based Broadband Wireless Access (BWA) system.
2. Description of the Related Art
Extensive research is being conducted to provide various Quality of Service (QoS) features with a data rate of about 100 Mbps in the advanced fourth-generation (4G) communication system. The 4G communication system is evolving to provide mobility, high data rate transmission, and high QoS in a BWA system such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN) system. Typical examples of the above system are based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16d and IEEE 802.16e.
The IEEE 802.16d based system and the BWA system use an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The IEEE 802.16d system considers only a fixed Subscriber Station (SS) and a single cell structure (i.e., the mobility of an SS is not considered). On the other hand, the IEEE 802.16e based system considers the mobility of an SS. When the mobility of an SS is considered, the SS will be referred to as a mobile station (MS).
FIG. 1 is a block diagram of a conventional BWA system.
Referring to FIG. 1, the BWA system has a multi-cell structure. The BWA system includes a cell 100, a cell 150, a base station (BS) 110 managing the cell 100, a BS 140 managing the cell 150, and a plurality of MSs 111, 113, 130, 151 and 153. The signal exchange between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153 is performed using an OFDM/OFDMA scheme. The MS 130 is located in a boundary region (i.e., a handover region) between the cells 100 and 150. When the MS 130 moves from the cell 100 of the BS 110 into the cell 150 of the BS 140 while communicating with the BS 110, the serving BS of the MS 130 is changed from the BS 110 to the BS 140.
In such a BWA system (IEEE 802.16 system), an Uplink (UL) random access channel is used as a ranging channel. Initial ranging, periodic ranging, and bandwidth request ranging are performed using the ranging channel. When an MS enters a network (network entry procedure) or loses its system (network) information, the MS performs initial ranging to obtain UL synchronization. In an initial ranging operation, a BS determines the exact time of arrival of a ranging signal from an MS, calculates a round trip delay between the BS and MS, and informs the MS of a timing offset corresponding to the round trip delay. Further, in the initial ranging operation, the BS allocates the MS a primary management CID and a basic CID that are required for MS to enter the network and receive a control message.
FIG. 2 is a diagram for explaining an initial ranging process in a conventional BWA system.
Referring to FIG. 2, when powered on, an MS 201 receives a Downlink (DL) preamble from a serving BS 203 for synchronization with the BS 203. In step 204 the MS 201 receives a DL-MAP message and a Downlink Channel Descriptor (DCD) message from the BS 203 and acquires information about a DL channel from the received messages. Next, the MS 201 receives an UL-MAP message and an Uplink Channel Descriptor (UCD) message from the BS 203 and acquires information about an initial ranging opportunity period, an UL channel, and initial ranging parameters.
In this way, the MS 201 acquires minimal information about the UL and DL channels, and then the MS 201 performs a basic access procedure using the acquired information to communicate with the BS 203. This basic access procedure is called an initial ranging procedure.
In detail, the MS 201 sends a ranging request (RNG-REQ) message to the BS 203 in step 205. In response to the RNG-REQ message, the BS 203 sends a ranging response (RNG-RSP) message to the MS 201 in step 207. In this way, an initial ranging procedure is performed.
Since the initial ranging procedure is performed before the MS 201 registers with the BS 203, both the MS 201 and the BS 203 do not have information about their connection. The MS 201 uses an initial ranging CID as a CID. The initial ranging CID has a predetermined value (e.g., 0×000). Since the initial ranging CID is commonly used for all MSs, the initial ranging CID is not individually handled.
In a BWA system, a plurality of different CIDs other than the initial ranging CID is used as shown in Table 1 below.
TABLE 1CIDValueInitial Ranging CID0x0000Basic CID0x0001~mPrimary management CIDm + 1~2mTransport CID2m + 1~FE9FMulticast CID0xFEA0~0x FEFE
Referring to Table 1, an initial ranging CID is used in an initial ranging procedure where an MS initially tries to access a BS as described above.
A Basic CID is a unique CID assigned to an MS by a BS. The basic CID can be used in place of a Media Access Control (MAC) address of the MS while the MS and the BS are connected. After an initial ranging procedure, the MS and the BS transmit and receive a control message using the basic CID.
A primary management CID is used mainly in a network entry process. The network entry process starts with an initial ranging procedure and includes a sequence of procedures, such as MS registration, service negotiation, and IP address allocation. The network entry process is a process required for an MS to register with a BS. In the network entry process, the primary management CID is mainly used for the BS to identify the MS. Further, the primary management CID is used for the BS and MS to transmit and receive an important control message. The primary CID is assigned to the MS by the BS and maintained while the BS and the MS are connected like in the case of the basic CID.
A transport CID is used for transmitting service data. When an MS requests a BS for a service after a network entry process, the BS assigns the MS a transport CID so as to transmit data corresponding to the requested service. Until the requested service is completely provided, the assigned transport CID is maintained to identify a connection established for the service. Each time the MS requests a service, the BS assigns the MS a transport CID. Therefore, when the MS simultaneously requests a plurality of services such as a voice call, an image service, and an Internet service, the MS can be assigned a plurality of transport CIDs. Unlike the transport CID, the primary management CID and the basic CID are assigned to an MS only once.
A multicast CID is used when the same data is simultaneously transmitted to a plurality of MSs. When a BS intends to transmit data using the multicast CID, the BS simultaneously assigns a multicast CID to a plurality of MSs and transmits the same data to the plurality of MSs. Then, each of the MSs determines that the data is destined for itself and receives the data.
Messages used for an initial ranging procedure will now be described.
Table 2 below shows syntax of an RNG-REQ message transmitted from an MS to a BS.
TABLE 2SyntaxSizeNoteRNG-REQ_Message_Format( ) {Management Message Type = 48 bitsDownlink Channel IDTLV encoded Information {variableTLV specificSS MAC AddressRequested Downlink Burst ProfileMAC VersionRanging AnomaliesAAS broadcast capability}}
Referring to Table 2, the RNG-REQ message includes a plurality of information entries. “Management Message Type” is equal to 4 and indicates the identity of the RNG-REQ message. “Downlink Channel ID” indicates a DL channel through which an UCD message including UL channel information is received. A TLV(Type/Lengthlvalue) encoded information field indicates encoded information including an SS Medium Access Control (MAC) Address, a Requested Downlink Burst Profile, an MAC Version, Ranging Anomalies and AAS(Adaptive Antenna System) Broadcast Capability. “SS MAC Address” is a MAC layer address of the MS and is used as an identifier of the MS. “Requested Downlink Burst Profile” is divided into a 0-3 bit section and a 4-7 bit section. In the 0-3 bit section, a Downlink Interval usage code (DIUC) is recorded for requesting formats required to receive and transmit physical channel signals (e.g., a modulation format and a error correcting format). The 4-7 bit section is a section for recording least significant bits (LSBs, 4 bits) of a Configuration Change Court field of the UCD message used for requesting ranging. The BS transmits a predetermined physical channel signal to the MS with reference to the information stored in the 0-3 bit section. “MAC version” indicates the version of a MC layer used by the MS. “Ranging Anomalies” includes information about whether the MS tries to access the BS at a maximum transmission (TX) power or a minimum TX power. When the BS instructs the MS to increase or decrease TX power to compensate for the TX power, time information, etc., the MS can use the Ranging Anomalies in response to the instruction of the BS. “AAS broadcast capability” indicates whether the MS is capable of receiving a broadcast message.
Table 3 below shows syntax of an RNG-RSP message transmitted from a BS to an MS.
TABLE 3SyntaxSizeNoteRNG-RSP_Message_Format( ) {Management Message Type = 58 bitsUplink Channel IDTLV encoded Information {variableTLV specificSS MAC Address6Downlink Operational2Burst ProfilePrimary Management CID2Basic CID2Ranging Status41 = continue2 = abort3 = success4 = rerangeTiming adjust4Power level adjust1Downlink frequency4Center Frequencyoverride(kHz) allowing an SSto perform an initialRanging Request again}}
Referring to Table 3, the RNG-RSP message includes various information. “Management Message Type field” has a value of ‘5’ to indicate that the present message is a Ranging Response message. “SS MAC Address field” contains a MAC layer address of the MS that will receive the RNG-RSP message. “Downlink Operational Burst Profile” is used as a response to the Requested Downlink Burst Profile of the RNG-REQ message from the MS and indicates a DIUC number that will be used by the BS. “Uplink Channel ID” indicates a UL channel for the MS. “Primary Management CID and a Basic CID are CIDs that are assigned to the MS by the BS in order to manage the connection between the BS and the MS while the MS receives a-service from the BS after a ranging procedure. “Ranging Status” indicates a response of the BS to a ranging request of the MS. When the Ranging Status has a value of ‘1’, it indicates the need to continue the ranging process. When the Ranging Status field has a value of ‘2’, it indicates the need to abort (stop) the ranging process. When the Ranging Status field has a value of ‘3’, it indicates the success of the ranging process. When the Ranging Status field has a value of ‘4’, it indicates the need to perform the ranging request again. “Timing Adjust” contains information that enables the MS to correct incorrect time information. “Power Level Adjust” contains information that enables the MS to adjust its TX/receiving (RX) power. “Downlink Frequency Override” is used to inform the MS of a frequency value of another channel, so that the MS can again perform an initial ranging request with another frequency when the Ranging Status is set to ‘2’ for indicating the need to abort the ranging process.
As shown in Table 3, in an initial ranging process, an MS is assigned a Primary Management CID and a Basic CID.
In an IEEE 802.16e system such as the conventional BWA system illustrated in FIG. 1, a fixed BS communicates directly with an MS and thus a reliable wireless communication link can be easily established between the BS and the MS. Since the BS is fixed in the IEEE 802.16e system, it is disadvantageous to construct a flexible wireless communication network. Communication services are not efficiently provided when traffic and call requests change.
To address this problem, a multi-hop relay scheme is used in the IEEE 802.16e system such as a cellular wireless communication system. In the multi-hop relay scheme, data is relayed from an origin to a destination through a fixed relay station (RS), a mobile RS, or an MS. A multi-hop relay wireless communication system can immediately reconstruct a network in response to a change in a communication environment and can use wireless network resources more efficiently. For example, a cell service coverage and a system capacity can be increased in the multi-hop relay wireless communication system. When channel conditions are not good between a BS and an MS, an RS can be disposed between the BS and the MS to establish a multi-hop relay path, thereby providing better wireless channels to the MS. Further, the multi-hop relay scheme can be used in a boundary region between cells where channel conditions are poor in order to provide high data rate channels and expand a cell service coverage area.
FIG. 3 is a diagram illustrating a BWA system using a multi-hop relay scheme to expand a BS service coverage area.
Referring to FIG. 3, the multi-hop relay BWA system has a multi-cell structure. The multi-hop relay BWA system includes a cell 300, a cell 340, a BS 310 managing the cell 300, a BS 350 managing the cell 340, a plurality of MSs 311 and 313 located within the cell 300, a plurality of MSs 321 and 323 located in a region 330 outside the cell 300 of the BS 310 and communicating with the BS 310, an RS 320 providing a multi-hop relay path between the BS 310 and the MSs 321 and 323 located in the region 330, a plurality of MSs 351, 353 and 355 located in the cell 340, a plurality of MSs 361 and 363 located in a region 370 outside the cell 340 of the BS 350 and communicating with the BS 350, and an RS 360 providing a multi-hop relay path between the BS 350 and the MSs 361 and 363 located in the region 370. An OFDM/OFDMA scheme is used for communication among the BS 310 and 350, the RS 320 and 360, and the MSs 311, 313, 321, 323, 351, 353, 355, 361, and 363.
Although the MSs 311 and 313 located in the cell 300 and the RS 320 can directly communicate with the BS 310, the MSs 321 and 323 located in the region 330 cannot directly communicate with the BS 310. Therefore, the RS 320 covers the region 330 to relay signals between the BS 310 and the MSs 321 and 323. That is, the MSs 321 and 323 can communicate with the BS 310 through the RS 320. Further, the RS 360 and the MSs 351, 353, and 355 located in the cell 340 can directly communicate with the BS 350, the MSs 361 and 363 located in the region 370 cannot directly communicate with the BS 350. Therefore, the RS 360 covers the region 370 to relay signals between the BS 350 and the MSs 361 and 363. That is, the MSs 361 and 363 can communicate with the BS 350 through the RS 360.
Since the RSs 320 and 360 are additionally used in the multi-hop relay BWA system shown in FIG. 3, the conventional initial ranging process shown in FIG. 2 cannot be used. For example, an RNG-REQ message of an MS located outside a cell coverage area may be delivered to a BS through an RS, and an RNG-RSP message of the BS may be delivered to the MS through the RS.
Fields of the RNG-REQ and RNG-RSP messages (refer to Tables 2 and 3) other than the Basic CID field and the Primary Management CID field cannot be processed by a BS communicating with an MS using a multi-hop relay scheme. For example, fields of the messages related to power adjustment and burst profiles used for transmitting and receiving physical channel signals are the most representative fields that cannot be processed by the BS. Since such fields should provide information about actual physical channels between an MS and an RS, the BS indirectly connected with the MS cannot process such fields.
For this reason, it can be proposed that an RS processes some fields of an RNG-REQ message of an MS that are related to the RS and then transmits only the remaining fields of the RNG-REQ message to a BS. For example, an RS can transmit only the SS MAC address of an RNG-REQ message of an MS to a BS and receives an assigned CID from the BS. Then, the RS can construct an RNG-RSP message using the CID received from the BS and information processed by the RS in order to send the RNG-RSP message to the MS.
However, this proposal disadvantageously requires an additional procedure in which the RS requests from the BS CID assignment and receives the assigned CID, thereby causing a time delay. Moreover, the time delay increases as the number of hops between the BS and the MS increases (in FIG. 3, a two-hop relay path is shown)
Therefore, there is a need for an apparatus and method for rapidly processing a ranging request of an MS communicating with an RS.